1. Field of the Invention
This invention relates to polyorganosiloxane gel materials. More particularly, this invention relates to the prevention of bubble formation in polyorganosiloxane gel materials at interfaces between the gel and substrates possessing potential nucleation sites where bubble formation is likely to occur.
2. Background Information
Cured polyorganosilanes exhibiting a gel type character are disclosed in the prior art, for example in U.S. Pat. No. 3,284,406 which issued to Nelson on Nov. 8, 1966, and in copending U.S. patent application Ser. No. 592,481, filed on Mar. 23, 1984. The polyorganosiloxane gel materials disclosed in these documents are prepared by reacting at least one polydiorganosiloxane containing terminal vinyl groups and a benzene soluble resinous organosiloxane copolymer containing SiO.sub.2, (CH.sub.3).sub.3 SiO.sub.1/2 and CH.sub.2 .dbd.CH(CH.sub.3).sub.2 SiO.sub.1/2 units with at least one polyorganosiloxane containing at least three silicon-bonded hydrogen atoms per molecule. The curable reaction mixture may also include at least one polydiorganosiloxane containing two silicon-bonded hydrogen atoms, one at each of the two terminal positions of the molecule, as a chain extending reactant. These types of polyorganosiloxane compositions are cured in the presence of a platinum-containing catalyst.
Cured polyorganosiloxane gels are characterized as being relatively soft and deformable in comparison to polyorganosiloxane elastomers and resins. The cured gels are typically too soft to exhibit hardness values on any of the Shore durometer scales. Polyorganosiloxane gels are often characterized in terms of penetration values that are typically expressed as the depth to which the surface of the gel is depressed by a specified loading applied for a specified time interval. Typical penetration values are within the range of from 1 to 10 mm or higher under a loading of 20 g. applied over a five second interval.
Polyorganosiloxane gels are used in numerous applications, including mammary prosthesis, as damping media and for the encapsulation of delicate electronic devices and circuits to protect them against damage by moisture and mechanical shocks. The use of polyorganosiloxanes as the light transmitting medium for a compressible optical waveguide device is disclosed in U.S. Pat. No. 4,128,299, which issued to J. Maher on Dec. 5, 1978, and is the copending U.S. patent application Ser. No. 592,481 referred to herinbefore.
For some of the foregoing applications, it is mandatory that the cured gel remain optically clear and free of bubbles or other defects that could alter the optical transmittance of the gel. It has been found when the gel is cured in contact with a substrate, such as the surface of a molded or extruded object containing contour variations or irregularities, these variations and irregularities can serve as nucleation sites at which gases dissolved or entrapped within the gel form bubbles. The gas can be air that becomes entrapped when the substrate is coated or otherwise contacted with the gel or can result from gaseous products generated within the gel during or following the curing reaction. The gels disclosed in the aforementioned patent to Nelson can generate gaseous hydrogen by cleavage of silicon-hydrogen bonds. Irrespective of the means by which gaseous materials arrive within the gel, bubble formation can occur when these gaseous materials contact potential nucleation sites present on the substrate. The relatively high elasticity of the gel allows bubbles to expand to the extent that they irreversibly alter the optical properties of the cured gel. The gel or the entire device must then be replaced, a costly and time consuming procedure.
One unsuccessful approach we used in trying to solve the problem of bubble formation was attempting to increase the degree of curing of the gel in the area adjacent to the substrate by coating the substrate with organosiloxane materials containing three or more silicon-bonded hydrogen atoms per molecule. A second unsuccessful approach was to coat the substrate with an organosiloxane primer considered useful for improving the adhesion of polyorganosiloxane compositions curable by a hydrosilation reaction.